Keynote Address

Coordination and Vision in Juggling

Free University of Amsterdam, Netherlands

Juggling is a complex rhythmic activity
requiring a high degree of coordination between hand movements and the objects
being kept aloft in the air or bounced against the floor. As the task constraints
of juggling are rather severe, particularly when juggling many objects, there
is little room for error, even though human motor performance is intrinsically
variable. The key question how this is accomplished is addressed along two lines.
First, it is discussed how coordination constraints such as frequency and phase
locking may lead to a reduction of the number of dimensions (i.e., principal
components) that are necessary for describing a particular juggling pattern
(e.g., the three-ball cascade), and thus for producing it. How the control problem
is being confronted depends on juggling speed, which affects both the number
of relevant principal components as well as the smoothness of the projections
of the signal onto the time evolution of the eigenvectors, particularly around
the catches. Besides this global and time-dependent aspect, it is shown that,
whereas the throws are highly reproducible, particularly the catches are used
to correct errors in the timing of the act (i.e., to stabilize the phase difference
between the objects).

Second, it is discussed to what extent,
and how, jugglers use advanced visual information for the control of catching.
In a one-ball juggling task jugglers look at the ball in the air at certain
preferred times (in the order of 360 ms before catching) rather than at preferred
places such as the zenith of the ball flight. In this task, expert jugglers
tend to look at the ball earlier than less experienced jugglers, allowing them
more time for making the interception. In ‘real’ juggling patterns (i.e., where
the number of objects exceeds the number of hands), such as the cascade, expert
jugglers often adopt a so-called "gaze through" (or "distant
stare") strategy, suggesting that the direction of gaze is restricted to
a small area and that information about the flights of the balls is being picked
up peripherally rather than centrally. If expert jugglers perform less standard
patterns, such as the reverse cascade, however, they may abandon the "gaze
through" strategy. If clear saccadic eye movements are present in intermediate
and expert jugglers alike, they are often mode locked to the objects being circulated.
(The effects of tempo and skill on these mode-locked patterns are currently
analyzed and will be discussed at the conference.)

In sum, jugglers appear to accomplish their
amazing feats by creating task-specific, tempo-dependent couplings between the
movements of the hands, objects and eyes, such that the errors induced by the
throws are annihilated